Passive identifiers capable of producing coded replies are known in the prior art. For example, in Cole et al. U.S. Pat. No. 3,706,094 issued Dec. 12, 1972 there is shown an electronic surveillance system including a label in the form of a surface acoustic wave device. Labels are coded by maufacturing cards with a full array of elements and by later severing the connections of an appropriate number of array elements from the main transmission line. Alternatively, labels manufactured in accordance with the Cole et al. patent may be coded during manufacture by omitting one or more array elements or by not connecting them to the transmission line.
Surveillance systems manufactured in accordance with the Cole et al. patent suffer from the disadvantage that severance of the connections of array elements from a transmission line is difficult to perform manually because of the small size of such elements. Labels coded during manufacture by omission or disconnection of elements lack the flexibility of labels coded by an ultimate use because severance of elements renders these labels incapable of replying with certain codes utilizing the severed elements. The Cole et al. labels suffer from the additional disadvantage that they are based upon surface wave devices, in which losses are extremely high compared with shear bulk wave devices.
It is a principal object of the present invention to provide an identification system having an identifier that may be coded manually by the ultimate user by imposing pressure upon a pad means intermediate an input transducer and an output transducer to interrupt communication between the transducers, and by removing pressure from the pad means to restore such communication. Because output transducers may be reconnected after they have been disconnected, the identification system of the invention is more flexible in varying codes than prior art systems in which output transducers are permanently disconnected from a delay line.
Another object of the invention is to produce an identification system having an identifier which is a microacoustic shear bulk wave device, thereby minimizing losses occurring during processing and increasing the maximum number of useful output transducers so that a larger number of different coded electronic replies may be produced in response to an electromagnetic signal.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification, taken in conjunction with the drawings.